An intricate and essential partnership is established early in life between the host and the intestinal microbiome, assuring the maintenance of microbiota homeostasis. Disturbance of this partnership is often associated with various pathological conditions including inflammatory bowel diseases (IBD) (Cho, I. & Blaser, M. J. The human microbiome: at the interface of health and disease. Nature reviews. Genetics 13, 260-270, doi:10.1038/nrg3182 (2012)). The microbiota of IBD patients are characterized by a decreased prevalence of protective microorganisms (i.e. Clostridium IXa and IV groups) and an expansion of detrimental bacteria (i.e. Enterobacteriaceae/Escherichia coli) (Manichanh, C., Borruel, N., Casellas, F. & Guarner, F. The gut microbiota in IBD. Nature reviews. Gastroenterology & hepatology 9, 599-608, doi:10.1038/nrgastro.2012.152 (2012).
Inflammatory Bowel Disease encompasses two principal conditions: ulcerative colitis (UC) and Crohn's disease (CD). Some patients have features of both subtypes and are classified as IBD-undefined (IBD-U) (Gastroenterology, 2007. 133(5): p. 1670-89). UC is defined by continuous mucosal inflammation starting in the rectum and restricted to the colon while CD inflammation can occur anywhere in the gastrointestinal tract, involves full thickness of the bowel wall and often with skip lesions (Gastroenterol Clin North Am, 2009. 38(4): p. 611-28; Gastroenterology, 2007. 133(5): p. 1670-89). Recent attempts to find new markers for IBD subtypes, such as conventional antibodies, have fared very poorly at differentiating colonic CD versus UC. As treatments and responses to medical therapies differ between CD and UC (J Pediatr Gastroenterol Nutr, 2010, S1-S13. The American journal of gastroenterology, 2011. 106 Suppl 1: p. S2-25; quiz S26. Gastroenterol Clin North Am, 2009. 38(4): p. 611-28) there is an urgent need for biomarkers to differentiate between CD and UC.
The primary tool used for both diagnosis and IBD management is endoscopy (World J Gastrointest Endosc, 2012. 4(6): p. 201-11). Endoscopy enables both visualization of the mucosa and access for mucosal biopsies to diagnose disease, to define disease extent and activity, and to monitor disease progression. The diagnostic accuracy from colonoscopy ranges from 60 to 74% (J Clin Pathol, 2002. 55: p. 955-60). Accurate and early diagnosis is essential for proper disease management. The goal of IBD treatment is to bring active disease into remission and to prevent follow-up relapse (flare-ups). The choice of treatment depends on disease type (CD versus UC), disease location, severity of disease, disease complications and individual host factors (e.g. nutritional and growth status, pubertal status, child's age and size, medication allergies) (J Pediatr Gastroenterol Nutr, 2010, S1-S13. The American journal of gastroenterology, 2011. 106 Suppl 1: p. S2-25; quiz S26. Gastroenterol Clin North Am, 2009. 38(4): p. 611-28). Current drug therapies consist of aminosalycylates, immune-modulators, corticosteroids, antibiotics and biological therapies (i.e. anti-TNFα monoclonal antibodies). The optimum therapeutic regimen for maintaining a disease free state still remains to be determined and the effectiveness of these drugs significantly differs between CD and UC (J Pediatr Gastroenterol Nutr, 2010, S1-S13. The American journal of gastroenterology, 2011. 106 Suppl 1: p. S2-25; quiz S26. Gastroenterol Clin North Am, 2009. 38(4): p. 611-28). For example, 5-aminosalicylic acid (5-ASA) drugs are moderately effective at inducing remission and preventing relapse in mild-to-moderate-active UC, while they are not recommended in the management of active CD (The American journal of gastroenterology, 2011. 106 Suppl 1: p. S2-25; quiz S26). Methotrexate is good evidence for use as maintenance therapy to prevent relapse in CD however, there is no evidence for its use in UC (The American journal of gastroenterology, 2011. 106 Suppl 1: p. S2-25; quiz S26). Greater doses of anti-TNFα therapies at more frequent intervals are being just now recognized to be required for successful treatment of severe UC as compared to standard treatment protocols in use for CD. One third of the cost associated with IBD is due to medical therapies (CCFC. 2008, report. p. 1-101) stressing the economic importance of an effective treatment and thereby an accurate diagnosis.
While the etiology of IBD is unknown, the gut microbiota is emerging as a key player in disease development and/or chronicity. Genome wide association studies in both adults and pediatric patients have identified novel IBD-associated genes but only define 25% of the genetic risk for developing IBD and excepting for very young infants (i.e. <2 years of age), no unique genes have been discovered that define pediatric IBD from adult-onset IBD. IBD is a complex polygenic disease involving multiple risk gene loci (Nature genetics, 2008. 40(8): p. 955-62. Nature genetics, 2009. 41(12): p. 1335-40. Nature genetics, 2010. 42(4): p. 332-7). These loci encode genes involved in innate and adaptive immunity, autophagy, and maintenance of epithelial barrier integrity for those genes that have known function. While these studies have shown us that multiple pathways are involved in the pathogenesis of IBD, we remain surprisingly ignorant on the root cause(s) and pathogenesis of IBD. A prevailing hypothesis is that IBD development is a consequence of functional abnormalities in the interplay between the intestinal microbiota and the host (World journal of gastroenterology: WJG, 2011. 17(5): p. 557-66). Some of the best evidence that the gut microbiota plays a key role in IBD comes from animal model studies (World journal of gastroenterology: WJG, 2011. 17(5): p. 557-66. Cell, 2007. 131(1): p. 33-45. Inflamm Bowel Dis, 2007. 13(12): p. 1457-66). Although the experimental animal models of IBD do not exactly mimic human IBD, these studies have shown that the development of the disease is dependent on the presence of resident bacteria (Cell, 2007. 131(1): p. 33-45. Inflamm Bowel Dis, 2007. 13(12): p. 1457-66). The loss of the transcriptional factor T-bet in mice, which regulates the differentiation and function of immune system cells, was shown to promote the microbiota to become colitogenic. Moreover, the induced colitis could be transmitted to other genetically intact hosts by vertical transfer of the colitogenic microbiota (Cell, 2007. 131(1): p. 33-45). Numerous studies have revealed alterations in the composition of the gut microbiota of patients with IBD (Proc Natl Acad Sci USA, 2007. 104(34): p. 13780-5. (9) Nature, 2010. 464(7285): p. 59-65. (10) Cell, 2012. 148(6): p. 1258-70; World journal of gastroenterology: WJG, 2011. 17(5): p. 557-66). However, we do not know what triggers IBD and the resulting gut microbiota dysbiosis and we have only a rudimentary understanding of the interplay between the gut microbiota and the host. Clearly, studies that longitudinally follow gut microbiota dysbiosis in humans during flare-ups and remissions could contribute important insights into the clinical significance of the gut microbiota composition.
IBD symptoms may include bloody diarrhea, abdominal pain, cramping, fatigue, various nutritional deficiencies including iron deficiency anemia, bone health problems and weight loss (Archives of disease in childhood, 2006). In children poor linear growth is also common. The onset of symptoms is slow, indolent and non-specific and so the disease may be present in certain regions of the bowel for very long periods of time prior to diagnosis. Following diagnosis, this chronic, life-long disease is characterized by episodes of flare-up and remission (quiescent, symptom-free state) (Gastroenterol Clin North Am, 2009. 38(4): p. 611-28; Archives of disease in childhood, 2006). The current therapeutic treatments aim to stop mucosal inflammation so as to maintain the quiescent period and to reduce flare-ups to reduce permanent bowel damage and alleviate the complications of disease. Corticosteroids (prednisone) remain a mainstay of treatment for IBD despite the well-known side effects of this medication (Journal of Crohn's & colitis, 2012. 6(4): p. 492-502). Alternatively, enteral nutrition (EN) is more commonly being used as a primary therapy in lieu of prednisone to induce CD remission (Current opinion in clinical nutrition and metabolic care, 2011. 14(5): p. 491-6). However, it is more difficult for most patients to adhere to these protocols that involve enteral formulas alone without eating foods for many weeks at a time. It is apparent that the microbiota composition correlates with disease and that an “abnormal” microbiota contributes to (if not triggers) mucosa alterations and immune system malfunctions (World journal of gastroenterology: WJG, 2011. 17(5): p. 557-66). It follows that interventions aimed at restoring microbiota equilibrium could promote health and/or prevent flare-up. Moreover, given that each patient is have a unique gut microbiota composition it follows that any interventions aimed at manipulating the gut microbiota should preferably be disease and patient-specific.
In view of the above there is a need for better diagnostic assays and treatments for the management of IBD.